[enhancement] Non-bits-typed parametrics should be supported in procs

[grebe]

What's hard to do? (limit 100 words)

Currently, the assumption that proc parametrics are bits-typed seems baked in multiple places.

Current best alternative workaround (limit 100 words)

Use bits types everywhere- if you want to use structs, pack and unpack at proc spawn boundaries.

Your view of the "best case XLS enhancement" (limit 100 words)

Any type should work as a proc parametric.

[mikex-oss]

Related (particularly if procs become struct + impl): #978

[richmckeever]

I think it's more likely we would steer users towards using generics, rather than a const instance of a struct, for a use case where they want this (which is what Rust seems to do). In other words:

proc P<T: type> {
   next() {
     // use T::value(), T::VALUE, T::VALUES.value, etc.
   }
}

[grebe]

Do you feel one of these more strongly than the other: "we shouldn't allow struct-typed parametrics" vs "it's OK as a stepping stone but not what we want ultimately"